From 695961f8ae7eedec245e4a214eec61a841aebc14 Mon Sep 17 00:00:00 2001
From: Sayed Kaif <metsw24@gmail.com>
Date: Thu, 23 Jul 2026 21:42:37 +0530
Subject: [PATCH] skip invalid fields in write_headers to prevent response
 splitting

---
 httplib.h    |  9 +++++++++
 test/test.cc | 45 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 54 insertions(+)

diff --git a/httplib.h b/httplib.h
index 4f77d37e..8558c566 100644
--- a/httplib.h
+++ b/httplib.h
@@ -7594,6 +7594,15 @@ inline ssize_t write_response_line(Stream &strm, int status) {
 inline ssize_t write_headers(Stream &strm, const Headers &headers) {
   ssize_t write_len = 0;
   for (const auto &x : headers) {
+    // Skip fields with invalid names or values to prevent response splitting
+    // via CR/LF injection, matching set_header(). The client validates request
+    // headers up front in check_and_write_headers, but the server passes
+    // res.headers straight to this writer, and res.headers is a public field
+    // an application can populate directly with request-derived values.
+    if (!fields::is_field_name(x.first) || !fields::is_field_value(x.second)) {
+      continue;
+    }
+
     std::string s;
     s = x.first;
     s += ": ";
diff --git a/test/test.cc b/test/test.cc
index 8358b581..8e4b8b75 100644
--- a/test/test.cc
+++ b/test/test.cc
@@ -7964,6 +7964,51 @@ TEST(ServerResponseSplittingTest, ChunkedTrailerCRLFInjection) {
   EXPECT_NE(std::string::npos, res.find("X-Safe: safe"));
 }
 
+TEST(ServerResponseSplittingTest, ResponseHeaderCRLFInjection) {
+  Server svr;
+  svr.Get("/injected-header", [&](const Request & /*req*/, Response &res) {
+    // res.headers is a public field an application can populate directly,
+    // bypassing set_header()'s validation (e.g. reflecting a request value).
+    // A valid header that must survive.
+    res.headers.emplace("X-Safe", "safe");
+    // Attacker-controlled value carrying CR/LF (response splitting).
+    res.headers.emplace("X-Reflected",
+                        "legit\r\nInjected-Header: pwned\r\nX-Evil: also");
+    // Attacker-controlled name carrying CR/LF.
+    res.headers.emplace("X-Bad\r\nSet-Cookie: a=1", "x");
+    res.set_content("body", "text/plain");
+  });
+
+  thread t = thread([&] { svr.listen(HOST, PORT); });
+  auto se = detail::scope_exit([&] {
+    svr.stop();
+    t.join();
+    ASSERT_FALSE(svr.is_running());
+  });
+
+  svr.wait_until_ready();
+
+  const std::string req = std::string("GET /injected-header HTTP/1.1\r\n") +
+                          "Host: " + HOST +
+                          "\r\n"
+                          "Connection: close\r\n"
+                          "\r\n";
+
+  std::string res;
+  ASSERT_TRUE(send_request(5, req, &res));
+
+  // The injected fields must not appear anywhere in the raw response.
+  EXPECT_EQ(std::string::npos, res.find("Injected-Header"));
+  EXPECT_EQ(std::string::npos, res.find("X-Evil"));
+  EXPECT_EQ(std::string::npos, res.find("Set-Cookie"));
+
+  // Invalid headers are dropped entirely, matching set_header().
+  EXPECT_EQ(std::string::npos, res.find("X-Reflected:"));
+
+  // The valid header still passes through.
+  EXPECT_NE(std::string::npos, res.find("X-Safe: safe"));
+}
+
 // Forward declaration: in split builds split.py strips `inline` and moves the
 // definition into httplib.cc, so detail::write_request_line is not visible from
 // the public httplib.h. Re-declaring it here lets the tests link against the